Method for producing highly pure, granular silicon in a fluidised bed

ABSTRACT

The present invention relates to a method for producing highly pure, granular silicon with a narrow particle-size distribution by decomposing silanes or halosilanes in a fluidised bed and epitaxially growing silicon on silicon seed particles, which method is characterised in that the gas containing silicon is supplied to the reaction chamber in an upward flow and the contents of the fluidised bed are separated in a continuous or discontinuous manner, whereby a particle stream from the fluidised bed is supplied to a separator mounted outside the fluidised bed, particles of the desired size are separated and undersized particles are returned to the fluidised bed. The invention also relates to a device and the use thereof for carrying out said method.

The invention relates to a method for producing highly pure, granularsilicon with a narrow particle-size distribution by decomposing silicicgases in a moving-bed or fluidised-bed reactor. Furthermore theinvention relates to a device and the use thereof for carrying out saidmethod.

Silicic gases as referred to herein are gases containing siliconcompounds or mixtures of silicon compounds which under the conditionsaccording to the invention can be decomposed in the gaseous phasedepositing silicon.

The growing demand for high-tech electronics and the efforts forreducing the consumption of fossil fuels by opening up alternativeenergy sources like solar energy has not only led to an increased demandfor semiconductor materials but particularly so to a dramatic increaseof the purity requirements on such substances.

Due to the good availability of the starting compounds as well as itsexcellent semiconductor qualities, the production of highly pure siliconfor the photovoltaic area and electronics is in the focus of interesthere.

A common method is the thermal decomposition of silanes or halosilanes,preferably carried out in a moving-bed or fluidised-bed reactor.

Methods wherein pure, elemental silicon is epitaxially growing onsilicon seed particles by thermally decomposing silanes or halosilanesin a fluidised bed are known for example from U.S. Pat. No. 3,012,861and U.S. Pat. No. 3,012,862.

In a fluidised-bed reactor, an initially stationary bed of solidparticles, in this case silicon seed particles, is fluidised by anupward flow of gas, i.e. said bed is turned into a liquid-like stateonce its volume flow exceeds a certain limit depending, among otherthings, on the density and the diameter of the particles. This limit iscalled loosening velocity. The advantages of such fluidisation are, forexample, the intensive mixing of particles and the large contact surfacebetween the solid and gaseous phases.

Such silicon-containing educts used for thermal decomposition aresilicon-hydrogen compounds, i.e. silanes, or halosilanes, which may bediluted by argon, helium, nitrogen or hydrogen.

The thermal decomposition of silanes is advantageous because unlikehalosilanes, silanes do not require additional reducing agents and nocorrosive gases like halogen hydrogens arise as by-products. Inaddition, the decomposition of silane occurs already in the range from500° C., whereas most halosilanes require at least 800° C., i.e. muchmore energy is required.

Many applications in the photovoltaic area or in semi-conductortechnology require highly pure, granular silicon with a narrowparticle-size distribution.

In this context, “highly pure” means a silicon content of at least99.99999%. The required average particle sizes are generally between 50and 5000 μm. A narrow particle-size distribution of the siliconparticles has the advantage that the further processing of such silicon,which is mainly done by remelting, requires less energy. U.S. Pat. No.3,012,862 describes a method for producing pure silicon by decomposinghalosilanes in the presence of the reducing agent in a fluidised bed.During the process of epitaxial growth, particles are formed which dueto their size are not fluidised any more, but deposit on the bottom ofthe bed in the reactor. This process is called segregation. Through aside outlet these large particles are conducted to an externalseparation and, if necessary, occurring fine material is returned intothe reaction zone. Such a method holds a considerable risk as asegregation on the bottom of the reactor can lead first to anagglomeration of the segregated bed contents, then during the furtherreaction to an agglomeration of the complete bed and finally to atermination of the process.

In U.S. Pat. No. 4,818,495, a construction is described wherein the gasrequired for fluidisation (fluidising gas) is blown into the reactorthrough a conical bottom, centrically arranged in which is a separationzone into which an upward gas stream (separation gas) is blown. JP-A06100312 depicts another such central outward transfer. When using acentral separation outlet with an upward gas feeding direction, theseparation gas is introduced into the reactor at a much higher velocitythan the fluidising gas. Because of the high velocity, the central flowof the separation gas is dominating and immediately attracts the gasbubbles forming of the fluidising gas in the reactor towards thiscentral flow. There may occur a decomposition of silane or halosilanesinside the gas bubbles, however, the silicon forming there occurs in theform of very fine particles which do not bond to the surface of the seedparticles. This process is called homogenous pyrolysis wherein generallyparticles of sizes below 50 μm are formed. As the settling speed of suchfine particles is very low they are carried out by the fluidising orseparation gas thus causing a loss which dramatically reduces the yieldof silicon deposited on seed particles (by so-called heterogeneouspyrolysis). When working without separation gas, produce with a verybroad particle-size distribution leaves the reactor through the centraloutlet. This means that in part particles are produced whose sizeexceeds the desired size by far. Furthermore some seed particles remainwithout any deposit.

Therefore there was a need to find a method wherein a high yield ofhighly pure, granular silicon with a narrow particle-size distributionsuitable for use in the photovoltaic area and in semi-conductortechnology can be obtained by decomposing silicic gases in a moving-bedor fluidised-bed reactor.

The invention relates therefore to a method for producing highly pure,granular silicon with a narrow particle-size distribution by decomposingsilanes or halosilanes in a fluidised bed and epitaxially growingsilicon on silicon seed particles, which method is characterised in thatthe gas containing silicon is supplied to the reaction chamber in anupward flow and the contents of the thus fluidised bed are separated ina continuous or discontinuous manner, whereby a particle stream from thefluidised bed is supplied to a separator mounted outside the fluidisedbed, particles of the desired size are separated and undersizedparticles are returned to the fluidised bed. The invention also relatesto a device and the use thereof for carrying out said method.

The special advantage of this method consists in that the fluidisedcontents of the bed has a narrow particle-size distribution, whichreduces the risk of segregation and defluidisation. Another advantage isthat the contents of the fluidised bed is finer because the particles ofthe desired size are carried out which encourages the formation ofsmaller gas bubbles. Smaller gas bubbles lead to a reduction of theunwanted dust formation thus enabling a high yield of silicon epitaxillygrown on seed particles.

The method according to the invention will now be described in moredetail with reference to the exemplary embodiment depicted in FIGS. 1and 2. This serves merely to enable a better understanding of theinvention without limiting the underlying principles of the invention toany extent.

FIG. 1 shows the reactor with the gas supply (1), the braking plate (2),the apertured bottom (3), the reaction chamber (4) with the heating (5).Outside the reaction chamber, there is the seal gas supply (6) towardsoutlet (7) surrounded by the ring chamber (8). The separation gas supply(9) leads towards the separation tube (10) with the subsequent finematerial feedback (11) discharging into the fine material cyclone (12).Below the cyclone is the fine material flap valve (13). The dust outlet(14) is located at the upper end of the reactor chamber. The bottom endof the separation tube (10) is connected to the finished productdischarge valve (15) of the collecting container (16). On top of thereaction chamber (4), the seed particle pipe (17) is arranged.

According to FIG. 2, the downward outlet (7) is located above theapertured bottom (3). The seal gas supply (6) leads to the ring chamber(8) with the propulsion jet bore holes (18), which chamber surrounds theoutlet (7).

According to FIG. 1, silicic gas is led to the reactor by means of thegas supply (1).

Besides silane SiH₄ and its higher homologues, as for example disilaneSi₂H₆, trisilane Si₃H₈, tetrasilane Si₄H₁₀ and hexasilane Si₆H₁₄,halosilanes of the general formula SiH_(m)Hal_((4−m)) can be used,wherein m can be an integer from zero to three and Hal can be chlorine,bromine or iodine. A mixture of such silicon compounds is also suitablefor the method according to the invention. Silicon compounds which arenot gaseous at room temperature can be thermally transferred into thegaseous phase for example.

Silane SiH₄, disilane Si₂H₆, trisilane Si₃H₈, tetrasilane Si₄H₁₀ andhexasilane Si₆H₁₄ are preferred, silane SiH₄ is particularly preferred.

The total volume percentage of silanes or halosilanes in the introducedgas can be for example between 1% and 100%, 5 to 20 volume percent arepreferred.

Furthermore the silicic gas may contain for example noble gases, e.g.argon or helium, nitrogen, other gases rendered inert under the reactionconditions or hydrogen or any mixture of such gases. Nitrogen andhydrogen are preferred, hydrogen is particularly preferred.

When using halosilanes, it must be ensured that per mol equivalenthalogen at least one mol equivalent hydrogen is provided in the silicicgas. Such hydrogen equivalents can originate for example from silanes,monohalosilanes, dihalosilanes or trihalosilanes or elementary hydrogen.

The entering impulse of the silicic gas flow is refracted by thesubsequent braking plate (2).

This is a common measure to ensure a steady penetration of the silicicgas flow serving as fluidising gas through the apertured bottom (3). Themethod according to the invention can also be carried out without such abraking plate. Other devices enabling the refraction of the impulse ofthe entering gas flow, e.g. deflection pipes or nozzles or anotherapertured bottom, are also possible.

The stream-in velocity of the introduced silicic gas should be betweenonce to tenfold the loosening velocity required for fluidising the bed,one and a half times to seven times the speed is preferred.

The temperature of the introduced silicic gas is preferably below thetemperature at which the silicic gas gets not yet decomposed. In thecase of silane this is a temperature of about 300° C. The minimumintroduction temperature is to be chosen such that it is at leastequivalent to the boiling point of the compound based on the usedpartial pressure of the silicic gas, however below the respective rangeof decomposition.

The pressure loss of the apertured bottom (3) completing the reactionzone at its bottom end can be adjusted in a way ensuring a steady feedstream into the reaction zone of the reactor (4).

The pressure at which the method according to the invention can becarried out is uncritical. It is preferred, however, to work atpressures ranging from 50 to 50000 mbar. 100 to 10000 mbar arepreferred, 200 to 6000 mbar are particularly preferred. All indicatedpressure values are absolute values referring to the pressure prevailingbehind the fluidized bed as seen in flow direction of the introducedgas.

The temperature of the apertured bottom should be selected such that itis below the decomposition range of the used silicon compounds duringthe process of epitaxial growth. Such range for silane is above approx.300° C.

Preferably the temperature of the apertured bottom is between 20 and300° C., particularly preferred between 200 and 250° C.

Higher temperatures can lead to silicon depositing in the aperturedbottom, thus obstructing the apertures and finally to a termination ofthe process. The apertured bottom can be provided with several aperturesensuring a homogeneous fluidisation of the above bed by the flow ofsilicic gas entering the reaction zone of the reactor.

The reactor (4) is surrounded by a heating, e.g. a resistance heating(5). The heating heats up the bed such that the temperature inside thereactor is above the decomposition range of the silicic gas, howeverbelow the melting point of silicon (1414° C.). The temperature of thereactor wall is for example 2 to 200° C. higher than the temperature inthe interior of the reactor, preferably between 5 and 80° C. higher.

When using silane, the advantageous temperature range in the interior ofthe reactor is between 500 and 1400° C., preferably between 600 to 1000°C. and particularly preferred between 620 to 800° C.

A part of the fluidised bed contents is introduced through the sideoutlet (7) into the separator (10), which is provided with separationgas by means of the separation gas supply (9).

Also several outlets can be provided for example. The outlet or theoutlets can also be integrated for example in the apertured bottom (3).

The separator can be for example a vertical upflow separator or a zigzagseparator. However, in all embodiments the separator should be locatedoutside the fluidised bed.

The separation gas can be a gas rendered inert under the said reactionconditions, for example a noble gas, e.g. argon or helium, nitrogen,hydrogen or any mixture thereof. Nitrogen and hydrogen are preferred,hydrogen is particularly preferred.

The fines carried by the separation gas are introduced through theascending pipe (11) into a cyclone (12), where it is deposited.

Other devices for the separation of fine material, such as filters, arealso suitable for the method according to the invention.

The feedback into the reactor is carried out by means of a fine materialfeedback pipeline provided with a flap valve (13) which opensautomatically if a sufficiently high column of fines has accumulated inthe fine material feedback pipeline.

It is also possible to collect the fine material in a collectingcontainer or to return it from the cyclone into the bed. Feedback of thefine material into the reaction zone is preferred.

Fine material is carried out of the reactor through the dust outlet(14).

Fresh seed particles can be introduced into the reaction zone throughthe seed particle pipe (17).

The seed particle pipe can also be arranged laterally.

Silicon particles in the separator (10) which are not led to the cyclone(12) fall down, pass through the open stop valve (15) and are collectedin a collecting container (16).

The desired particle size can be adjusted according to the usualprocedures by controlling the flow of separation gas.

According to FIG. 2, silicon particles can leave the fluidised bed ofthe reactor (4) through the lateral downward outlet or connector element(7). The penetrating amount can be controlled by means of a control gasflow introduced into the outlet (7) through the supply (6) and the ringchamber (8) as well as the bore holes (18) that are inclined at an anglerelative to the outlet direction. This allows also to stop the flowcompletely.

This enables separation to be carried out in a continuous ordiscontinuous manner, a continuous separation is preferred.

For separation, an outward transfer rate is preferred corresponding to0.1 to 15 times, preferably 0.5 to 5 times and particularly preferred 1to 3 times the bed contents.

Control gases can be for example gases rendered inert under the reactionconditions, for example noble gases, e.g. argon or helium, nitrogen,hydrogen or any mixture thereof. Nitrogen and hydrogen are preferred,hydrogen is particularly preferred.

The amount of penetrating particles can also be controlled for exampleby a mechanical control valve. The outlet (7) can also be integrated inthe apertured bottom (3) for example.

In the course of the process based on the method according to theinvention, particles with an average desired size between 50 and 5000 μmare obtained in the collecting container (16). The particle-sizedistribution of the product can be such that at least 90 weight percentof the separated particles have a grain size differing from the desiredsize by maximum 20%. A particle-size distribution where at least 90weight percent of the separated particles have a grain size differingfrom the desired size by maximum 10% is preferred.

Preferably the method according to the invention is integrated into acomplex method for producing silane and highly pure silicon.

It is particularly preferred that the method according to the inventionbe integrated into a method for producing silane and/or highly puresilicon comprising the following steps:

-   -   1. Trichlorosilane synthesis and subsequent isolation of the        produced trichlorosilane by distillation and recycling of the        unreacted silicon tetrachloride, and, if desired, the unreacted        hydrogen;    -   2. Disproportionation of trichlorosilane to silane and silicon        tetrachloride through the intermediate stages of dichlorosilane        and monochlorosilane on basic catalysts, preferably catalysts        containing amino groups, carried out in two apparatuses or in        one, and recirculation of the produced silicon coming out as a        high-boiling component into the first reaction area.    -   3. Further use of the silane of the purity achieved in the        preceding step, or purification of the silane until the purity        required for the intended purpose is achieved, preferably by        distillation, particularly preferred by distillation under        pressure.    -   4. Thermal decomposition of silane to obtain highly pure silicon        and method according to the invention.

Starting parameters: Reactor diameter 52 mm Mass of bed 800 g Diameterof seed particles 200 μm Reaction temperature 650° C. Pressure 1 barSilane 10 Vol % Hydrogen 90 Vol % Gas velocity 1.1 m/s

COMPARATIVE EXAMPLE 1

(Reactor without a Separator)

A laboratory reactor is run in continuous operation. Permanently seedparticles (200 μm) are introduced into the reactor and product particlesare discharged. At a feeding rate of approx. 5 g/h, 113 g/h productparticles with an average diameter of 740 μm is produced (silaneconversion: 39.7%, dust selectivity: 14.3% u/u_(mf):2.6). The particlesizes range from 200 to 1600 μm (90% passing).

EXAMPLE 2

(Reactor with Separator)

The reactor is run in continuous operation as specified in Example 1. Ata seed particle feeding rate of approx. 0.8 g/h, approx. 105 g/h siliconis produced. With a separation grain of 1000 μm, an average diameter ofbed particles of 770 μm and an average diameter of product particles of1022 μm is obtained (at a reactor discharge rate of approx. 1300 g/h)(silane conversion: 41% dust selectivity: 14.4% u/u_(mf):2.8).

1. A method for producing highly pure, granular silicon with a narrowparticle-size distribution by decomposing silanes or halosilanes in afluidized bed and epitaxially growing silicon on silicon seed particles,the method comprising: providing a reaction chamber; providing aseparator; providing a separation gas supply; providing a cycloneelement; supplying a gas containing silicon is supplied to the to saidreaction chamber in an upward flow; separating contents of the fluidizedbed in a continuous manner, whereby a particle stream from the fluidizedbed is supplied through at least one side outlet into said separator,said separator being mounted outside the fluidized bed, said separatorbeing provided with separation gas via said separation gas supply,wherein particles of a desired size are separated via said separator,said separation gas including fine material after said particles areseparated via said separator; controlling a flow of said particle streamthrough said at least one side outlet from said reaction chamber by agas flow entering said at least one side outlet via another outlet, saidgas flow being opposite to an outlet flow direction of said particlestream; and introducing said fine material in said separation gas intosaid cyclone element via an ascending pipe, said fine material beingseparated from said separation gas via said cyclone element.
 2. A methodaccording to claim 1, wherein after separation undersized particles arereturned to the fluidized bed.
 3. A method according to claim 1, whereinthe desired size of the particles obtained by the separation process isbetween 50 and 5000 μm.
 4. A method according to claim 1, wherein atleast 90% of the particles obtained by the separation process have agrain size differing from the desired particle size by maximum 20%.
 5. Amethod according to claim 1, wherein the flow of said gas containingsilicion is introduced into the reactor chamber through a reactor bottomwith more than one aperture.
 6. A method according to claim 1, wherein apressure prevailing in a portion of said reactor chamber located behindthe fluidized bed as seen in flow direction of said gas containingsilicion, is between 50 and 50000 mbar.
 7. A method according to claim1, wherein the separation is carried out in a vertical upflow separator.8. A method according to claim 1, wherein the separation is carried outin a zigzag separator.
 9. A method according to claim 1, wherein saidfine material is separated from the separation gas in a dust separator.10. A method according to claim 1, wherein said fine material isdelivered to said reaction chamber.
 11. A method according to claim 1,wherein said gas containing silicon contains silane SiH₄.
 12. A methodaccording to claim 1, wherein said gas containing silicon containshydrogen.
 13. A method according to claim 1, wherein silicon isproduced, said silicon being used in the photovoltaic area.
 14. A methodaccording to claim 1, wherein silicon is produced, said silicon beingused in the manufacture of electronic components.
 15. A method forproducing highly pure, granular silicon with a narrow particle-sizedistribution by decomposing silanes or halosilanes in a fluidized bedand epitaxially growing silicon on silicon seed particles, the methodcomprising: providing a reaction chamber, said reaction chamber having abottom portion with at least one aperture; providing a separatorincluding an ascending tube, said tube being connected to said reactionchamber; providing a separation gas supply; providing a connectorelement connected to said separation gas supply, said separator beingconnected to said reaction chamber via said connector element; providinga fluidized bed; supplying a flow of gas containing silicon to saidreaction chamber, said flow of gas containing silicon being supplied ina longitudinal direction of said reaction chamber; separating contentsof said gas containing silicon in said fluidized bed in a continuousmanner such that a flow of particles from the fluidized bed is deliveredto said separator via, said connector element, said separator beinglocated at a position outside said fluidized bed, said separation gassupply supplying separation gas to said separator such that said flow ofparticles mixes with said separation gas, said separator separating saidparticles mixed in said separation gas such that said separation gascontains a desired size of particles; controlling said flow of saidparticles from said reaction chamber to said connector element based ona controlled feeding of a control gas supplied to said connectorelement; and providing a fine particle separation means for separatingsaid desired size of particles from said separation gas; delivering saidseparation gas containing said desired size of particles to said fineparticle separation means via said tube; separating said desired size ofparticles from said separation gas with said fine particle separationmeans after said separation containing said desired size of particles isdelivered to said fine particle separation means to form separateddesired sized particles.
 16. A method according to claim 15, whereinsaid separated desired sized particles delivered to said fluidized bed.17. A method according to claim 15, wherein said separated desired sizedparticles have a size between 50 and 5000 μm.
 18. A method according toclaim 15, wherein a pressure prevailing in a portion of said reactorchamber located behind the fluidized bed as seen in flow direction ofsaid gas containing silicion, is between 50 and 50000 mbar.
 19. A methodfor producing highly pure, granular silicon with a narrow particle-sizedistribution by decomposing silanes or halosilanes in a fluidized bedand epitaxially growing silicon on silicon seed particles, the methodcomprising: providing a reaction chamber, said reaction chamber having aside surface and a bottom portion with at least one aperture, said sidesurface having a reaction chamber opening; providing a separatorincluding an ascending tube, said tube having a tube surface defining atube opening, said separator being located at a spaced location fromsaid reaction chamber; providing a separation gas supply; providing aconnector element connected to said separation gas supply, saidconnector element being connected to said separator and said reactionchamber, wherein said reaction chamber opening is in communication withsaid tube opening; providing a fluidized bed; supplying a flow of gascontaining silicon to said reaction chamber, said flow of gas containingsilicon being supplied in a longitudinal direction of said reactionchamber; separating contents of said gas containing silicon in saidfluidized bed in a continuous manner such that a flow of particles fromthe fluidized bed is delivered to said separator via said connectorelement, said separator being located at a position outside saidfluidized bed, said separation gas supply supplying separation gas tosaid separator such that said flow of particles mixes with saidseparation gas, said separator separating said particles mixed in saidseparation gas such that said separation gas contains a desired size ofparticles; controlling said flow of said particles from said reactionchamber to said connector element based on a controlled feeding of acontrol gas supplied to said connector element; and providing a fineparticle separation means for separating said desired size of particlesfrom said separation gas; delivering said separation gas containing saiddesired size of particles to said fine particle separation means viasaid tube such that said desired size of particles are separated fromsaid separation gas with said fine particle separation means.